1. Field of the Invention
The present invention relates to an optical integrated circuit; and, more particularly, to a method for preparing an improved hybrid optical integrated circuit which is capable of accommodating optical waveguides, optical devices, such as light emitting devices or light receiving devices, and optical fibers in an effective manner.
2. Description of the Prior Art
As the amount of information has been dramatically increased in a modern society, in order to transmit such great amount of the information, an optical integrated circuit has been suggested as a core technique in view of the performance and price for an optical transmission system. Such an optical integrated circuit includes two types: optoelectronic integrated circuit(OEIC); and a hybrid integrated circuit(HIC). In the OEIC, an optical waveguide, a light emitting device and a light receiving device are prepared by using the same material and optically coupled to each other on a monolithic wafer which is made of a compound semiconductor, for example, InP. While, in the HIC, a light emitting device and a light receiving device are combined on a wafer by using a surface mounting technique. In view of the conventional techniques, the HIC is more realistic to be implemented than the OEIC.
Various efforts have been addressed to prepare a hybrid optical integrated circuit in which an active optical devices, such as a light emitting device and a light receiving device, are mounted on a wafer together with an optical waveguide. Referring to FIG. 1, there is shown a hybrid optical integrated circuit described in U.S. Pat. No. 4,735,677, issued to Masao Kawachi et al, entitled "Method for fabricating hybrid optical integrated circuit." As shown in FIG. 1, the hybrid optical integrated circuit includes a high-silica glass optical waveguide 141 formed on a silicon substrate 140, a semiconductor laser 147 as a light emitting device, a semiconductor photodetector 148 as a light receiving device, guides 142 for aligning the semiconductor laser 147, guides 143 for aligning optical fibers 149. The high-silica optical waveguide 141 has a Y-branch shape which defines three end faces 144 to 146. The semiconductor laser 147 is positioned between the guides 142 on the silicon substrate 140 and coupled to a first end face 144. The photodetector 148 is mounted on the silicon substrate 140 and coupled to a second end face 145. The optical fiber 149 is located between the guides 143 and coupled to a third end face 146.
The optical waveguide 141 is generally integrated on the silicon substrate 140 and the semiconductor laser 147 and the optical fiber 149 are aligned to the first and third end faces 144 and 146 of the optical waveguide 141 by using the semiconductor laser alignment guides 142 and the optical fiber alignment guides 143, respectively. Usually, high silica optical waveguide 141 should have a thickness of several tens of microns to meet a single mode condition. In this case, the etching process for fully defining or forming the optical waveguide 141 significantly attacks the etching mask layer also so that the dimension of the optical waveguide 141 and alignment guides 142 and 143 may be altered in micron range. Furthermore, the semiconductor laser 147 should have to machined with submicron accuracy in order to be precisely aligned by the semiconductor laser alignment guides 142, while it is so difficult to treat a compound semiconductor material, for example, InP, GaAs or the like, used in fabricating the semiconductor laser 147 in a submicron-level precision due to the brittleness thereof.
The waveguide device used for transmitting light within optical devices and optical integrated circuits must have a low light transmission loss and a comparable mode size with optical devices to be interconnected with thereof. In some cases, it should have electro-optic or thermo-electric effects, in order to implement passive and active waveguide devices.
Since single crystal silicon widely used in semiconductor integrated circuit have a higher transmittance in a wavelength range of 1.2 to 1.6 micron, it is possible to use the silicon layers as an optical waveguide in the above wavelength range. FIG. 2 shows a cross-sectional view illustrating a conventional rib-type SOI (Silicon On Insulator) optical waveguide which is typically formed on a SOI wafer. The SOI wafer has been fabricated by the silicon direct bonding or the separation by implantation of oxygen (SIMOS) methods. The SOI waveguide structure includes a buffer layer 151 formed on a silicon substrate 150, a core layer 152 and a cladding layer 153. Typically, the buffer layer 151 formed at a thickness of 1 to 2 .mu.m has been made of a silicon oxide layer. The core layer 152 has been made of a single crystal silicon layer having a thickness of 2 to 10 .mu.m. And a silicon oxide layer formed by oxidizing the surface of the core layer 152 is used as the cladding layer 153. In order that light is guided within the core layer 152 of the SOI waveguide and satisfies a single mode condition in the vertical and horizontal directions, the width (W) 155 of a rib, the height (H) of the core layer 157 and the height (rH) 154 of a slab are given by: EQU r.ltoreq.0.5
##EQU1##
Accordingly, to obtain the maximum optical coupling efficiency when the SOI waveguide is coupled to the laser and the optical fiber, the thickness 157 of the core layer 152, the width 155 of the rib and the height 156 of the rib should be controlled under the condition of satisfying the above equation.
As an example of conventional hybrid optical integrated circuits using the SOI waveguides, FIG. 3 shows an alignment between a waveguide and a laser("ASOC.TM.-A silicon-based integrated optical manufacturing technology," Tim Bestwick et al, Proceedings of the 48th ECTC, 1998, pp 566-571). As shown in FIG. 3, a rib-type SOI waveguide 162 is formed by etching an undesired portion of a single crystal silicon layer 164 which is isolated from a silicon substrate by a buried oxide layer 163 and a laser 161 coupled to the SOI waveguide 162 is mounted on a recess 168 formed when the single crystal silicon layer 164 is etched to form the rib. Accordingly, a guide for aligning the laser 161 to the SOI waveguide 162 is made of a single crystal silicon thin film and a horizontal alignment between the laser 161 and the SOI waveguide 162 is achieved by attaching the laser 161 to both sidewalls 166 and 169 of the recess 168.
As the prior art employing a silica waveguide, the above-mentioned optical integrated circuit using a waveguide thin film as mechanical stops of optical devices, still has had a problem itself in that semiconductor laser is to be mechanically processed. Since SOI waveguide thin film, the thickness of which is only about 10 .mu.m, cannot be used as an alignment guide of an optical fiber, the thickness of which is about 125 .mu.m. As the same as the prior art using a silica waveguide, any optical fiber aligner except for the SOI waveguide thin film such as anisotropically etched silicon V-groove is necessary. However, in the case of making a structure for optical fiber alignment by other means but SOI waveguide thin film, a lot of optical coupling loss may be caused between the optical fiber and the waveguide due to the misalignment therebetween.
A shortcoming of a SOI waveguide is that a lot of Fresnel loss may occur in the front and back facet of the SOI waveguide when light is incident from the atmosphere to the SOI waveguide or radiated from SOI waveguide to the atmosphere because a refractive index of the silicon layer (about 3.5) used as a core material is much larger than that of the atmosphere (1). It is necessary to form anti-reflection film in the front and back facet of the waveguide in order to lessen Fresnel loss. It is desirable that the most appropriate anti-reflection film has a refractive index of 1.87 and its thickness has n.lambda./4 (n: integer) of the wavelength in the material.
FIG. 4 is a cross-sectional view illustrating an optical coupling between a SOI waveguide and a surface receiving optic detector and forming an anti-reflection film. An optical integrated circuit in FIG. 4 includes the steps of; forming a mirror facet 172 of about 54 angles of inclination to the surface of substrate by etching a single crystal silicon layer 177 of location opposite to a SOI waveguide 170; forming an aluminum reflection film 171 on the mirror facet 172; and commonly forming anti-reflection film 173 on the waveguide's sidewall 175 and the aluminum reflection film 171. At this time, incident light 176 propagating in parallel to the surface of the substrate, emitted from the waveguide 170, is reflected on the aluminum reflection film 171 and then enters into an optic detector 174 mounted over the single crystal silicon layer 177. In this art, there is a tendency of preventing the Fresnel loss in the waveguide facet 175 and mirror facet 172 by simultaneously forming the anti-reflection film 173 on the mirror facet 172 coated with the aluminum reflection film 171 between the mirror facet 172 and the waveguide facet 175. However, the formation of the anti-reflection film 173 is carried out after the deposition of aluminum 171. Therefore, there are such problems that the anti-reflection film must be formed at a temperature below 400.degree. C. so that a metal layer such as an aluminum layer may not be damaged. Accordingly, such a good anti-reflection film as formed at a temperature higher than 600.degree. C., such as LPCVD (low pressure chemical vapor deposition) silicon nitride layer having a refractive index of about 2 which is useful for an anti-reflection film, cannot be used.
There is a problem in a hybrid optical integrated circuit using a silica waveguide and a SOI waveguide in the prior art that the optical device itself must be precisely processed in order to align the optical device to an optical device alignment guide comprised of a waveguide thin film itself. As described above, the waveguide thin film itself cannot be used as the optical fiber alignment guide in the prior art for manufacturing the hybrid optical integrated circuit using the SOI waveguide. Under such a reason, it has been necessary to form such a special optical fiber alignment guide as an anisotropically etched silicon V-groove and at this time, a misalignment between the optical fiber alignment guide and the waveguide and a difficulty in forming the anti-reflection film appropriate for the SOI waveguide may occur.